It's rumored Baron Strucker could be eyed to be the secondary villain of the film with Hydra returning. Given the events of Cap 2 are said to lead into the Avengers (and it looks like the film is about Cap's past coming to haunt his present) it seems likely HYDRA would return. If this is the case I wouldn't be surprised if the main baddy from Cap 2 (Zemo, Zola, Strucker) forms the masters of evil. It would make sense story wise to top a team of heroes with a team of villains, it would be a good introduction to Quicksilver and Scarlett Witch without making them feel forced in, and finally it would be a good way to show how much of threat Ultron is by having a pre corrupted version of him own them.

Come on, it's not like the movie could have been anywhere near as action packed as the last if they used a villain who didn't become a threat until near the end.